Manager with a bad attitude - no respect for customers and staff

I am a regular at KwaNobuhle Shoprite buying mainly bags of Potatoes ( in bulk ) for my Take-Away business - selling chips. My problems first started when the price of potatoes went down to R25/bag - I was told that I must only buy one bag. ( On a normal day I use at least 15 bags of potatoes and on busy days I use 25 bags or more.) There was no sign that said " only one bag per customer " . This rule is not app**** everyday . Yesterday the price of potatoes went up from R31.99 to R36.99 and I loaded 10 bags onto the trolley and went to the tills - nobody approached and notified me that I could only buy one bag. When I came to the till the cashier told me that I can only buy one bag of potatoes. This was ridiculous because the potatoes were not even on special , in fact they were the most expensive since I started buying them. I asked for the manager who told that I could only buy one bag. I asked him why there is no notice of this limit at the Potatoes stand - he told me that they do not sell to Hawkers. I explained to him that I do not sell bags of potatoes - I sell chips. He then told me that he is the Manager and can therefore decide how many bags of potatoes each customer can buy. I again asked him why there was no such sign at the Potatoes Stand and he walked away leaving me at the till with no answers and no solution to my problem.